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Q. QN. Sequential Logic Circuits. Unlike combinational logic circuits, the output of sequential logic circuits not only depends on current inputs but also on the past sequence of inputs.

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Sequential Logic Circuits


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    1. Q QN Sequential Logic Circuits • Unlike combinational logic circuits, the output of sequential logic circuits not only depends on current inputs but also on the past sequence of inputs. • Sequential circuits are constructed using combinational logic and a number of memory elements with some or all of the memory outputs fed back into the combinational logic forming a feedback path or loop. • A very simple sequential circuit with no inputs created using inverters to form a feedback loop: When this circuit is powered up it randomly outputs Q = 0 or Q =1

    2. Combinational outputs Memory outputs Combinational logic Memory elements Inputs Sequential Logic Circuits Sequential circuit = Combinational logic + Memory Elements Current State of A sequential Circuit: Value stored in memory elements (value of state variables). State transition: A change in the stored values in memory elements thus changing the sequential circuit from one state to another state.

    3. Q Memory element Memory Element Output: stored single-bit value command Sequential Circuit Buliding Blocks:Generic Memory Elements • A Memory Element: A logic device that can remember a single-bit value indefinitely, or change its value on command from its inputs. • The output Q of the memory element represents the value stored in the memory element. This is also called the state variable of the memory elements. A memory element can be in one of two possible states: • Q = 0 (the memory element has 0 stored), also said be in state 0. • Q =1 (the memory element has 1 stored), also said to be in state 1. • The commands to the memory element formed by its input(s) may include: • Set: Store 1 (Q=1) in the memory element. • Reset: Store 0 (Q=0) in the memory element. • Flip: Change stored value from 0 to 1 or from 1 to 0. • Hold value: Memory value does not change. • Memory Element state transition: A change in the stored value from 0 to 1, or from 1 to 0 such as that caused by a flip command.

    4. The State of A sequential Circuit • A state variable in a sequential circuit represents the single-bit variable Q stored in a memory element in circuit. • Each memory element may be in state 0 or state 1 depending on the current value stored in the memory element. • The State of A sequential Circuit: • The collection of all state variables (memory element stored values) that at any time contain all the information about the past necessary to account for the circuit’s future behavior. • A sequential circuit that contains n memory elements could be in one of a maximum of 2n states at any given time depending on the stored values in the memory elements. • Sequential Circuit State transition: A change in the stored values in memory elements thus changing the sequential circuit from one state to another.

    5. 1 Clock signal 0 Clock Cycle Time Falling edges of the clock Rising edges of the clock Clock Signals & Synchronous Sequential Circuits • A clock signal is a periodic square wave that indefinitely switches values from 0 to 1 and 1 to 0 at fixed intervals. • Clock cycle time or clock period: The time interval between two consecutive rising or falling edges of the clock. • Clock Frequency = 1 / clock cycle time(measured in cycles per second or Hz) • Example: Clock cycle time = 1ms clock frequency = 1000Hz • Synchronous Sequential Circuits: Sequential circuits that have a clock signal as one of its inputs: • All state transitions in such circuits occur only when the clock value is either 0 or 1 or happen at the rising or falling edges of the clock depending on the type of memory elements used in the circuit.

    6. Sequential Circuit Memory Elements: Latches, Flip-Flops • Latches and flip-flops are the basic single-bit memory elements used to build sequential circuit with one or two inputs/outputs, designed using individual logic gates and feedback loops. • Latches: • The output of a latch depends on its current inputs and on its previous inputs and its change of state can happen at any time when its inputs change. • Flip-Flop: • The output of a flip-flop also depends on current and previous input but the change in output (change of state or state transition) occurs at specific times determined by a clock input.

    7. Sequential Circuit Memory Elements: Latches, Flip-Flops • Latches: • S-R Latch • S-R Latch With Enable • D-Latch • Flip-Flops: • Edge-Triggered D Flip-Flop • Master/Slave S-R Flip-Flop • Master/Slave J-K Flip-Flop • Edge-Triggered J-K Flip-Flop • T Flip-Flop With Enable

    8. R Q QN S S-R Latch • An S-R (set-reset) latch can be built using two NOR-gates forming a feedback loop. • The output of the S-R latch depends on current as well as previous inputs or state, and its state (value stored) can change as soon as its inputs change. Function Table S R Q QN 0 0 last Q Last QN 0 1 0 1 1 0 1 0 1 1 0 0 Circuit

    9. S Q Enable C QN R S Q C Q R S-R Latch With Enable • Since the S-R latch is responsive to its inputs at all times an enable line C is used to disable or enable state transitions. • Behaves similar to a regular S-R latch when enable C=1 Circuit Function Table S R C Q QN 0 0 1 last Q last QN 0 1 1 0 1 1 0 1 1 0 1 1 1 0 0 x x 0 last Q last QN Logic Symbol

    10. D Q C Q D-Latch • Similar to S-R latch with an enable line, but both S, R are generated from one input D (data) and an inverter. • Stores the value of its input D when enable C =1. C D Q QN 1 0 0 1 1 1 1 0 0 x Last Q Last QN Function Table D Q C QN Logic Symbol Circuit

    11. Master Latch Slave Latch QM D Q D Q D Q QN D Q C Q C Q CLK Q CLK D CLK Q QN 0 0 1 1 1 0 x 0 Last Q Last QN x x Last Q Last QN Clock Circuit Edge-Triggered D Flip-Flop • Uses a pair of D latches and inverters. • Similar in behavior to a D latch except that output and state changes happen at the rising or falling edge of an input clock. • A D Flip-Flop triggered on the rising edge of the clock is given by: Logic Symbol Function Table

    12. Master Latch Slave Latch QM S Q S Q S Q C C QN Q Q R R R CLK S R C Q QN x x 0 last Q last QN 0 0 last Q last QN 0 1 0 1 1 0 1 0 1 1 undef. undef. S Q C Q R Master/Slave S-R Flip-Flop • S-R latches are substituted for the D latches in the negative-edge triggered D flip flop Circuit Function Table Logic Symbol

    13. Circuit Master Latch Slave Latch QM Q S Q S Q J C C QN Q Q R R K CLK J K C Q QN x x 0 last Q last QN 0 0 last Q last QN 0 1 0 1 1 0 1 0 1 1 last QN last Q S Q C Q R Master/Slave J-K Flip-Flop • Solves the problem in the problem when both S=R=1 • When J=K=1 the last state is inverted. Function Table Logic Symbol

    14. Circuit J Q D Q K QN CLK Q CLK J K C Q QN x x 0 last Q last QN x x 1 last Q last QN 0 0 last Q last QN 0 1 0 1 1 0 1 0 1 1 last QN last Q j Q CLK Q k Edge Triggered J-K Flip-Flop • Created from an edge-triggered D flip-flop Logic Symbol Function Table

    15. Q D Q En CLK QN Q T EN T En Q QN x 0 last Q last QN 1 last QN last Q Q j Q T CLK QN Q k T Flip-Flop With Enable • Changes state on every clock cycle (rising edge of T). Circuit OR Function Table